Oxidative cleavage of alcohols by cerium (IV) and chromic acid

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Iowa State University Capstones, Theses and
Retrospective Theses and Dissertations
Dissertations
1969
Oxidative cleavage of alcohols by cerium (IV) and
chromic acid
Paul Michael Nave
Iowa State University
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Recommended Citation
Nave, Paul Michael, "Oxidative cleavage of alcohols by cerium (IV) and chromic acid " (1969).Retrospective Theses and Dissertations.
4135.
https://lib.dr.iastate.edu/rtd/4135
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70-13,615
NAVE, Paul Michael, 1943-
OXIDATIVE CLEAVAGE OF ALCOHOLS BY CERIUMCIV)
AND CHROMIC ACID.
Iowa State University, Ph.D., 1969
Chemistry, organic
University Microfilms, Inc.. Ann Arbor, Michigan
THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED
OXIDATIVE CLEAVAGE OF ALCOHOLS BY CERIUM(IV)
AND CHROMIC ACID
by
Paul Michael Nave
A Dissertation Submitted to the
Graduate Faculty in Partial Fulfillment of
The Requirements for the Degree of
DOCTOR OF PHILOSOPHY
Major Subject: Organic Chemistry
Approved:
Signature was redacted for privacy.
Th' Charge of Major Work
Signature was redacted for privacy.
of Major Department
Signature was redacted for privacy.
of Graduat liege
wa State University
Ames, Iowa
1969
ii
TABLE OF CONTENTS
INTRODUCTION
HISTORICAL
RESULTS AND DISCUSSION
EXPERIMENTAL
SUMMARY
BIBLIOGRAPHY
ACKNOWLEDGMENTS
1
INTRODUCTION
The mechanism of the cerium(IV) oxidation of benzyl
alcohols to benzaldehyde has been studied recently in this
laboratory (1, 2). It was concluded that oxidation proceeds
via rapid 1:1 alcohol-cerium(IV) complex formation followed
by a rate determining unimolecular decomposition of the
complex. It was found that ring substitution causes little
change in the equilibrium constant of complex formation,
but does have an effect on the decomposition step. The
effect of the substituent can be correlated by a Hammett
ap treatment to give a p value of -1.07+0.28 using
values
Whereas benzyl alcohols give good yields of benzalde­
hyde (3), with most other alcohols, aldehyde or ketone forma­
tion is very poor (4, 5). One of the most prominent oxida­
tion routes is C-C cleavage. This cleavage reaction has
been reported by several workers (5, 6, 7); however, a
detailed mechanistic study was not made. The Hammett op
relationship has proved extremely useful as a mechanistic
probe, and it was felt that this would be the best approach
for studying the cleavage reaction. Thus, a series of
2-aryl-l-phenylethanols
OH
2
was prepared and subjected to competitive oxidation by
eerie ammonium nitrate in order to determine the effect of
the substituent on the departing benzyl group.
During the course of this work, it became apparent that
the results could serve as a model for cleavage by a one-
electron process and be compared with the results obtained
using more complex oxidants in which cleavage is always
accompanied by other modes of oxidation. Since cleavage in
chromic acid oxidations has been a subject of controversy
(vide infra), the study of this process was undertaken.
The results obtained for the competitive oxidations of
substituted 1,2-diphenylethanols with eerie ammonium nitrate
and chromic acid were compared with the results obtained
in an internal competition in which there were two modes of
cleavage from the same molecule, in order to gain more in­
formation about the overall mechanism.
3
HISTORICAL
Oxidative Cleavage of Alcohols
Monohydric alcohols
There is much confusion in the literature over the role
of cleavage in oxidative processes, especially in that of
hydroxy compounds. Almost all of the common metal ion
oxidants are known to cause C-C bond cleavage, at least in
part, for appropriately substituted alcohols and glycols.
In some cases, yields of cleavage products as high as 100%
can be obtained. Often, cleavage is a serious side route
causing low yields of a desired carbonyl product.
In those cases in which 100% cleavage of certain
monohydric alcohols has been reported, all are by oxidants
whose inorganic chemistry has shown them to be one-electron
oxidants. Jones and Waters (8) reported that phenyl-tert-
butylcarbinol was oxidized by vanadium(V), a one-electron
oxidant, to give almost exclusively benzaldehyde and products
derived from the tert-butyl fragment. Because of the one-
electron nature of the oxidant, a free radical mechanism
was proposed;
This same alcohol was reported by Mosher, Clement, and Hillard
to undergo virtually total cleavage by cerium(IV), another
4
one-electron oxidant, to give benzaldehyde and tert-
butanol (6). A free radical mechanism has also been proposed
in oxidative cleavage by cobalt(III), a powerful one-
electron oxidant (9). In this case, tertiary alcohols have
been shown to give cleavage products in similar ratios
to those of cleavage of the corresponding alkoxy radicals.
It is in the oxidation of alcohols by those oxidants
in which both one-electron and two-electron processes can
take place that the confusion exists over the role of
cleavage. Oxidation by chromic acid and lead tetraacetate
are the two most notable examples.
In the case of chromic acid oxidations, both types of
oxidation can take place because of the presence of the
relatively unstable intermediate chromium oxidation states,
chromium(V) and chromium(IV), in addition to the chromium(VI)
ion. In 1944 Mosher and Whitmore (10) found that phenyl-
tert-butylcarbinol gives products from a cleavage reaction
as well as the normal pivalophenone. In 1956, Hampton, Leo,
and Westheimer (11) established that one of the intermediate
ions is responsible for cleavage by showing that manganous
or cerous ions, which remove chromium(IV) as soon as it is
formed, drastically reduced the extent of cleavage. By
varying the conditions used, yields of cleavage products as
high as 67% could be obtained. On the basis of this upper
limit of cleavage product yields, the authors favored the
5
mechanism in Scheme 1 in which chromium(V) is responsible
for cleavage.
Cr(VI)+ RCHOHR' -»• Cr(IV) + ketone
Cr(IV) + Cr(VI) 2 Cr(V)
2 Cr(V) + 2 RCHOHR' 2 Cr(III) + 2 RCHO + 2 R'OH
Scheme 1. Proposed mechanism showing chromium(V)
giving rise to 67% cleavage ("Westheimer
mechanism")
One should note that cleavage by this mechanism would be a
two-electron process. They also presented the mechanism
shown in Scheme 2 in which chromium(IV) would cause
cleavage; however, they did not favor this mechanism since
it would account for only 33% cleavage.
Cr(VI) + RCHOHR' -> Cr{IV) + ketone
Cr(IV) + RCHOHR' -> RCHO + R'• + Cr(III)
Cr(VI) -r R'. + HgO ^ Cr(V) + R'OH + h"*"
Cr(V) + RCHOHR' -> Cr(Iir) + ketone
Scheme 2. Proposed mechanism showing chromium(IV)
giving rise to 33% cleavage
Earlier, Mosher and Whitmore had tentatively proposed an
oxonium ion intermediate as presented in Scheme 3 to account
for both cleavage and ketone formation.
6
+ 0
OH :0: R-C-R'
_L
CrOg
R- C —R' R—C—R'
HOAc +
à H O
HC-R'
Scheme 3. Oxonium ion mechanism of Mosher and
Whitmore
As support of this mechanism, Mosher, Clement, and Hillard
(6) in 1964 offered the evidence that when the R-group is
varied as £-substituted phenyl groups, there is little change
in the ratio of cleavage products to ketone. At the same
time, these authors supported the Westheimer mechanism.
How both of these mechanisms may be simultaneously satisfied
is not made clear. The only argument against a free radical
process is that isobutylene and 2,2,3,3-tetramethylbutane,
expected from the tert-butyl radical, were not found. Waters
has suggested, on the other hand, that since cleavage of
this type is observed with vanadium(V), a one-electron process
involving chromium{IV) and leading to a free radical is a
strong possibility (12). Recently, Rocek and Radkowsky
have shown that it is possible in certain cases to experi­
mentally distinguish between reactions of chromium(V) and
chromium(IV) (7). They found that cyclobutanol is readily
cleaved by chromium(IV), but is relatively unreactive toward
chromium(V). Wiberg and Schafer (13) have found by other
methods that in many cases chromium(V) behaves very much like